Certain applications of cathode ray tube (CRT) displays require highly accurate and reliable beam positioning with absolute registration. An example of this requirement exists for aircraft Head Up Displays (HUD). A HUD requires that certain CRT generated symbols be projected optically against the pilot's outside view of the real world, with accurate absolute registration. This accuracy must be maintained over production variation, environmental conditions, and over the life of the equipment. Present methods generally rely on use of high quality, expensive, low drift analog designs and components. In addition, periodic maintenance testing and calibration are also employed. Internal circuit monitoring is usually limited to a general test of the deflection amplifiers output with a known sample input. A much more desirable situation would involve monitoring the entire analog circuit path which controls the CRT beam position. A most ideal situation would be to drive the beam in a closed loop manner somewhat like a servo-mechanism.
Over a period of more than two decades, the military have proven the value of the HUD concept. All new generation fighter aircraft are equipped with HUD as standard issue. A major reason that the concept has been slow to penetrate the civil sector is cost. It is considered that the potential market in civil aviation for a cost effective HUD system has been substantial for a considerable number of years.